Development of sustainable methods of making transportation fuels and surfactants from renewable resources is becoming increasingly important, due to the desirability of decreasing dependence on petroleum resources. Carbohydrates obtained from biomass are renewable and readily available, and there has been much focus on producing fuel and other useful materials from biomass-derived starting materials. Other readily available starting materials include simple sugars, glycerol and the oxidation product of glycerol, dihydroxyacetone (DHA). Compounds suitable for use as transportation fuels generally include C8-C15 hydrocarbons. Commercially useful surfactants may comprise from about 10 to about 22 carbon atoms. Therefore, in order to produce transportation fuels and surfactants from biomass-derived starting materials, the carbon chain length of the starting material must be increased. One common method of achieving this is through the aldol reaction, which forms a carbon-carbon bond between an aldehyde and a ketone. Cellulosics, sugars and glycerin can readily be converted to suitable reagents for the aldol reaction. For example, cellulose and glucose can be converted to hydroxymethylfurfural (HMF). Dihydroxyacetone can be formed from glycerol.
Use of the aldol reaction with reagents derivable from cellulosics and sugars has been described previously (see, e.g., Huber et al., Science, vol. 208, Jun. 3, 2005, pp. 1446-1450). However, the reactions resulted in a range of carbon chain lengths, and thus exhibited poor selectivity. In addition, the reactions required high temperatures (approximately 100° C.), and the use of an organic solvent in addition to water. Most important, the reaction was shown to be unsuccessful with ketone reagents other than acetone. All of these factors contribute to making the process less suitable for large-scale industrial use.
The use of a zinc-proline (Zn(Pro)2) catalyst has been shown to be successful in certain types of aldol reactions, and addresses some of the above drawbacks. Zinc-proline catalysts result in higher selectivity, and have been shown to work with DHA as a ketone reagent. To date, however, zinc-proline catalysts have shown only to successfully catalyze reactions between ketones and aromatic aldehydes (for example, benzaldehydes) and not between ketones and reagents derived from biomass sources, such as HMF. Although able to catalyze reactions in aqueous solvents and at lower temperatures, the reactions described to date using zinc-proline catalysts have required the addition of an organic co-solvent, such as tetrahydrofuran (THF).
There exists a need, therefore, for a method of increasing carbon chain length via the aldol reaction that utilizes reagents derived from biomass sources, is highly selective, can be performed at room temperature and does not require the use of an organic co-solvent, thus making the process more suitable for large-scale production with specificity. There exists a further need for additional catalysts that allow the aldol reaction to proceed under conditions suitable for large-scale use.